Azo bond formation on metal surfaces

Article abstract of DOI:10.1002/anie.202011858

The formation of azo compounds via redox cross-coupling of nitroarenes and arylamines, challenging in solution phase chemistry, is achieved by on-surface chemistry. Reaction products are analyzed with a cryogenic scanning tunneling microscope (STM) and X-ray photoelectron spectroscopy (XPS). By using well-designed precursors containing both an amino and a nitro functionality, azo polymers are prepared on surface via highly efficient nitro-amino cross-coupling. Experiments conducted on other substrates and surface orientations reveal that the metal surface has a significant effect on the reaction efficiency. The reaction was further found to proceed from partially oxidized/reduced precursors in dimerization reactions, shedding light on the mechanism that was studied by DFT calculations.

Full text of DOI:10.1002/anie.202011858

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
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Accepted Article
Title: Azo bond formation on metal surfaces
Authors: Xiangzhi Meng, Henning Klaasen, Lena Viergutz, Bertram
Schulze Lammers, Melanie C. Witteler, Harry Mönig, Saeed
Amirjalayer, Lacheng Liu, Johannes Neugebauer, Hong-Ying
Gao, Armido Studer, and Harald Fuchs
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10.1002/anie.202011858
Angewandte Chemie International Edition
RESEARCH ARTICLE
Azo bond formation on metal surfaces
Xiangzhi Meng,[a, c, #] Henning Klaasen,[b, #] Lena Viergutz,[b] Bertram Schulze Lammers,[a] Melanie C.
Witteler,[b, d] Harry Mönig,[a] Saeed Amirjalayer,[a, d] Lacheng Liu,[a] Johannes Neugebauer,[b, d] Hong-
Ying Gao,*[a, e] Armido Studer,*[b, c] and Harald Fuchs*[a, c]
Dedication
[a]
Dr. X. Meng, B. S. Lammers, Dr. H. Mönig, Dr. S. Amirjalayer, L. Liu, Dr. H. Y. Gao, Prof. H. Fuchs.
Physikalisches Institut
Westfälische Wilhelms-Universität Münster
Wilhelm-Klemm-Straße 10, 48149 Münster, Germany
and
Center for Nanotechnology
Heisenbergstraße 11, 48149 Münster, Germany.
E-mail: fuchsh@uni-muenster.de (H. Fuchs).
gaohongying@tju.edu.cn (H. Y. Gao);
Dr. H. Klaasen, L. Viergutz, M. C. Witteler, Prof. A. Studer, Prof. J. Neugebauer.
Organisch-Chemisches Institut
[b]
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster, Germany
E-mail: studer@uni-muenster.de (A. Studer)
Dr. X. Meng, Prof. A. Studer, Prof. H. Fuchs.
Institution Center for Soft Nanoscience
Busso-Peus-Straße 10, 48149 Münster, Germany
M. C. Witteler, S. Amirjalayer, Prof. J. Neugebauer
Center for Multiscale Theory and Computation
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster, Germany
Dr. H. Y. Gao.
[c]
[d]
[e]
[#]
School of Chemical Engineering and Technology
Tianjin University
Tianjin 300072, China
Theses authors contributed equally.
Supporting information for this article is given via a link at the end of the document.
Abstract: The formation of azo compounds via redox cross-coupling
of nitroarenes and arylamines, challenging in solution phase
chemistry, is achieved by on-surface chemistry. Reaction products
are analyzed with a cryogenic scanning tunneling microscope (STM)
and X-ray photoelectron spectroscopy (XPS). By using well-designed
precursors containing both an amino and a nitro functionality, azo
polymers are prepared on surface via highly efficient nitro-amino
cross-coupling. Experiments conducted on other substrates and
surface orientations reveal that the metal surface has a significant
effect on the reaction efficiency. The reaction was further found to
proceed from partially oxidized/reduced precursors in dimerization
reactions, shedding light on the mechanism that was further studied
by DFT calculations.
Introduction
During last decade, on-surface chemistry has attracted
extensive attention as a bottom-up strategy for the synthesis of
materials with interesting properties and functions. Various
organic reactions have been successfully realized on metal
surfaces[1-4] and most of them aim for carbon-carbon bond
formation. Only a few carbon-nitrogen and nitrogen-nitrogen bond
forming processes have been reported, such as imine,[5–7]
Scheme 1. (a) Investigated coupling pathways and b) bifunctional
monomers towards on-surface azo group formation.
1
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RESEARCH ARTICLE
imide-,[8] azo
group
formation[9–11]
and
azide-alkyne
by XPS analysis, see below) resulting in cyclic oligomers (Fig. 1c)
and long chains (Fig. 1d). According to the STM images, most of
the azo groups were formed via nitro-amino coupling (black
arrows in Fig. 1c and d), as estimated by the angle at the
connection points between the monomeric units. Few azo groups
(~15%) were formed through nitro-nitro homocoupling (white
arrows in Fig. 1b and d) and azo groups resulting from amino-
amino coupling were rarely observed. For NBPA, the reaction
efficiency in Scheme 1a could be ranked as path 1 >> path 2 >
path 3. In line with this finding, reductive homocoupling of 4,4’’-
dinitro-p-terphenyl (DNTP), a monomeric precursor designed to
follow reaction pathway 2, showed low efficiency on Ag(111)
(Fig. S1) and only short azo chains were obtained. To further
validate the selectivity, monofunctional aromatic precursors were
tested in cross-dimerization experiments (see SI). In agreement
with the reactivity trend observed for NBPA, nitro-amino and nitro-
nitro couplings were noted, whereas products derived from
arylamine homocouplings following path 3 were rarely identified
(Fig. S2-S5). In case of the attempted aniline homocouplings, we
identified amides coordinating to surface adatoms. Importantly, it
was found that electron withdrawing groups attached to the aniline
reduce the amount of these adatoms. We assume that amine
metalation suppresses oxidative homocoupling. On the other
hand, the adatoms might lead to a more reactive substrate and
be beneficial for side reactions such as reductive homocoupling
of nitro groups or branching. Further, it was found that rigid
compounds engage in these cross-couplings more efficiently.
Monomers with fixed conformation and planar structure showed a
better cross-coupling selectivity (Fig. S3) and the statistical
analysis revealed that NBPA, substituted with an electron
withdrawing group and low degree of conformational flexibility,
afforded the highest tendency toward the redox coupling product
(Fig. S6). Homo-dimerization of anilines following path 3 was
rarely observed without the presence of the nitro functionality in
accordance with previously reported studies[18,19] (Fig. S7).
cycloaddition.[12,13] Among these N-based functional groups, the
azo group shows valuable properties in photo switching.[14–16] In
solution phase chemistry, azo compounds are obtained through
an azo coupling using diazonium salts, but the cross-coupling of
nitro and amino compounds remains challenging. Herein, on-
surface azo formation by nitroarene-arylamine redox cross-
coupling (path 1 in Scheme 1a) is successfully realized on the
Ag(111) surface under ultrahigh vacuum (UHV) conditions. On-
surface azo formation via reductive nitroarene or oxidative
arylamine homocoupling (paths 2 and 3 in Scheme 1a) shows
poor efficiency. Low-temperature scanning tunneling microscopy
(LT-STM) and X-ray photoelectron spectroscopy (XPS) were
applied for product analysis. The chemical structures of organic
precursors used are depicted in Scheme 1b (for their synthesis,
see the Supporting Information, SI). By using well-designed
precursors, azo polymers and alternating azo oligomers are
accessible on-surface. In addition, the effect of the metal surface
on the azo formation is investigated.
Figure 1. Azo formation on Ag(111) surface. (a) STM image of NBPA self-
assembled on Ag(111) surface after deposition at RT. Inset: zoomed-in
STM image of (a). (b-d) STM images after annealing the sample at 393 K
for 30 min. Black arrow: azo groups formed by nitro-amino coupling. Green
arrow: azo chain. Inset in (d): zoomed-in image of azo group formed by
nitro-nitro coupling (white arrows). Set point: (a and inset) U = –1.2 V,
I = 80 pA. (b-d) U = –1.4 V, I = 70 pA. Size of inset in a: 2.0x2.0 nm2, d:
2.3x2.3 nm2
Results and Discussion
To distinguish the three possible coupling reactions
(paths 1-3 in Scheme 1a), the unsymmetrical bifunctional 4’-
nitrobiphenyl-3-amine (NBPA) was studied on the Ag(111)
surface (Fig. 1). After adsorption on Ag(111) kept at room
temperature (RT), NBPA assembled into
a well-ordered
monolayer (Fig. 1a). The self-assembly exhibits hexagonal
honeycomb motif composed by hexamer units, in which H-
bonding is assumed between the nitro and amino groups.[17] After
annealing (393 K), azo groups were formed (as further supported
Figure 2. Synthesis of linear azo polymers on Ag(111). (a) STM image of
NTPA assembly. (b) Zoomed-in STM image of (a). (c) STM image of
polymers obtained after annealing the sample at 440 K for 30 min. (d)
Zoomed-in image of the polymer. Set point: (a) U = –2 V, I = 50 pA; (b)
U = –1 V, I = 50 pA; (c, d) U = –1 V, I = 70 pA.
2
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To achieve long azo polymer chains and guided by the
identified reactivity trend, the linear 4”-nitro-p-terphenyl-4-amine
(NTPA) bearing an amino as well as a nitro functionality was
prepared. The stability of NTPA during evaporation was
confirmed with in situ mass spectrometry. Upon adsorption on the
Ag(111) surface at RT, NTPA molecules assembled into regular
sieve structures (Fig. 2a and b). H-bonds are assumed to connect
neighboring compounds. Resulting from the substrate symmetry,
the neighboring NTPA molecules are not perfectly perpendicular
to each other within the assembly. After annealing at 440 K for
30 min, terphenyl polymers linked by azo bonds were formed
(Fig. 2c). The high-resolution STM image showed that azo groups
were formed with trans-geometry (Fig. 2d). The distance between
the centers of the nearest terphenyl groups is 1.51±0.03 nm,
which agrees with a theoretical model (Fig. S8) and an ex situ
prepared model azo compound that further exhibits a similar STM
topography (Fig. S9). Tip manipulations confirmed the covalent
bonding between the polymer units (Fig. S10).
Next, we tested the effect of the substrate on the reaction
outcome and NTPA was investigated on various surfaces. For
comparison, STM results recorded after annealing at 440 K are
shown in Fig. 3. Images showing the self-assembled structure of
NTPA after deposition and the reaction products at other
temperatures are presented in the SI (Fig. S11). On Ag(110),
polymers with a similar contrast and length of the recurring motif
as observed on Ag(111) were obtained after annealing at 440 K
of NTPA caused the thermodynamically unstable (1x3)
reconstruction of Au(110) (Fig. S11j). Annealing at 440 K led to a
more pronounced formation of the (1x3) reconstruction as well as
to polymerization of NTPA inside the (1x3) channels (Fig. 3e and
f). Some depressions (green arrow in Fig. 3f) were observed on
the polymers, which were ascribed to the missing atoms
underneath. A similar phenomenon was also noted on Ag(110)
(Fig. S11d). The distance between two recurring moieties
indicates the same outcome as found on Ag(111). Overall, the
(110) surfaces provide a distinct confinement for the products,
which were observed with higher order than on the (111) surfaces.
Finally, reactivity of NTPA was studied on Cu(111). Instead of the
azo formation, activation of N-H, N-O and C-H bonds occurred.
After deposition and at different annealing temperatures, clusters
and short chains connected via coordination bonds (Fig. 3g and
h, Fig. S11 m-p) were the main products, in agreement with
previously described dehydrogenation of aniline derivatives on
Cu(111).[18,19] Overall, our experiments indicate that the azo
formation reaction is highly influenced by the metal type as well
as the surface structure.
Attempting alternating copolymerization, DNTP and
biphenyl-4,4’-diamine (BPDA) were co-adsorbed on Ag(111) (Fig.
4). After deposition, ordered and disordered phases were
observed, denoted by black dashed rectangle and white dashed
ellipse in Fig. 4a. Consistent with previous works,[18,19] the BPDAs
were found connected by adatoms (Fig. 4b and c, also see
Fig. S7). In the disordered phases, azo connected oligomers were
already formed at RT (white and blue arrow in Fig. 4c), as also
found for other monomers, usually after long time annealing
(Fig. S12 and S13). Annealing (343 K) improved reaction
efficiency (Fig. 4d) and alternating azo chains with up to five units
were identified (Fig. 4e and f). A strictly alternating polymerization
was difficult, as the nitro-nitro coupling of DNTP would introduce
defects. Furthermore, BPDA could easily desorb from the surface
ꢀ
(Fig. 3a and b). They tend to align along the [110] direction of the
surface, but multiple defects (green arrows in Fig. 3b) were noted
at the azo linkers. On Au(111), NTPA self-assembled in long 1D
chains along the reconstructed herringbones after deposition (Fig.
S11e). Coupling did not occur at 440 K and the self-assembled
structure remained (Fig. 3c and d). The distance between the
recurring motif in the chain is 1.71±0.03 nm, clearly indicating
linkage via H-bonding. Au(110) was more reactive and deposition
Figure 3. NTPA at different surfaces. (a, b) STM images recorded after annealing a sub-monolayer of NTPA on Ag(110) at 440 K. (c, d) STM images
recorded after annealing a sub-monolayer of NTPA on Au(111) at 440 K. (e, f) STM images recorded after annealing a sub-monolayer of NTPA on
Au(110) 440 K. (g, h) STM images recorded after annealing a sub-monolayer of NTPA on Cu(111) at 440 K. Set points: (a) U = -1.5 V, I = 160 pA; (b)
U = -0.2 V, I = 370 pA; (c, d) U = -1.1 V, I = 47 pA; (e, f) U = -1.3 V, I = 130 pA; (g) U = -0.85 V, I = 400 pA; (h) U = -0.14 V, I = 570 pA.
3
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at high temperature, perturbing the 1:1 stoichiometry of the two
reaction partners. To achieve longer alternating polymers,
strategies of surface confinement and specially designed
precursors need to be considered.
390 K, the amino peak vanished and two new peaks were
observed at 400.1 eV and 397.7 eV. In consistency with the STM
results, the peak at 400.1 eV indicates the formation of azo
groups.[23–25] The N 1s peaks observed on Ag(111) agree well with
Figure 5. N 1s XP spectra recorded on clean Ag(111) surface, NTPA
monolayer adsorbed on cold Ag(111) (170 K), NTPA monolayer annealed
at 390 K for 30 min, and multilayers deposited onto Ag(111) held at RT.
Dashed line: azo peak.
Figure 4. Towards alternating polymers on Ag(111) surface. (a, b, c) Co-
adsorption of DNTP and BPDA. a) Overview image with ordered (black
rectangle) and disordered (white ellipse) structures. b) Zoomed-in ordered
assembly. c) Zoomed-in disordered phase. Blue arrows: azo bond formed
by nitro and amino. White arrow: terphenyl chains. Black arrow: BPDA
chains via N-Ag-N bonds. (d-f) STM images recorded after annealing at
343 K for 30 min. d) Overview image with surface hollows. e) Zoomed-in
image of alternating chains. Red arrow: azo bonds. f) High resolution
image of azo-linkage. Set point: (a, b) U = –1.7 V, I = 50 pA; (c) U = –
1.7 V, I = 60 pA (d) U = –1 V, I = 50 pA; (e,f) U = –1.3 V, I = 45 pA.
those of powders and reported data (Table S1). The peak at
397.7 eV is ascribed to deoxygenated nitro[26] and the
dehydrogenated amino groups,[7,27,28] before azo group formation.
On the multilayer sample, the N 1s peak arising from the nitro
groups is observed at 406 eV.[21,29,30] The absence of the peak of
nitro groups in the monolayer samples could result from the strong
background as well as the easy reduction of nitro groups on the
silver surface (O 1s spectra in Fig. S14). The signals of intact nitro
and amine groups in a monolayer of NTPA were only observed
on the Au(111) surface (Fig. S14), revealing the very low activity
of these functional groups in line with their poor reaction efficiency
found in STM experiments. The XP spectra concede a possible
mechanistic model in which the nitro groups are deoxygenated
and reduced stepwise, while the amino groups are partially
oxidized and metalated. This causes the net comproportionation
before azo formation. We therefore assumed that
arylhydroxylamines, as a result of the weak N–O bond, should
easily be metalated like amines and thus lead to reactive
intermediates of the sequence. Indeed, by using N-(naphthalene-
1-yl)hydroxylamine as starting material, dimerization towards 1,1’-
azonaphthalene at 383 K was achieved at Ag(111) (Fig. S15), in
line with our mechanistic model as discussed below. Furthermore,
nitrosoarenes could either be intermediates or act as precursors
To understand the chemical states, XPS experiments were
conducted. The spectra recorded were analyzed using
a
commercial processing software (CasaXPS). Fig. 5 shows the
N 1s spectra recorded on NTPA samples prepared at different
conditions. In the region of N 1s, the clean silver surface shows a
strong background. The spectrum of the clean surface was fitted
by two optimized components (not shown), whose peak positions
and full width at half maximums were used to reckon the surface
contribution in the analysis of the spectra on other samples. Since
the coupling between nitro and amino groups can be performed
at RT (Fig. S12 and S13), a monolayer was deposited onto
Ag(111) at 170 K to avoid a partial reaction during the XPS
measurement. In the XP spectrum, a peak at a binding energy of
399.6 eV was observed and could be assigned to the amino
groups.[20-22] Unfortunately, even under these conditions the nitro
group could not be observed in the spectrum. After annealing at
4
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Figure 6. Calculated energy profile of the reaction of aniline and nitrobenzene on Ag(111) and corresponding structures. Two possible pathways via
the most energetically favorable intermediate structures are highlighted in green color. All alternative routes are shown as dashed black lines.
for intermediates of our reaction sequence. To validate our
assumption of stepwise reduction of the nitro group we tested
the intermediates with the lowest energies are highlighted in
green color. As first steps, an H atom and an O atom are removed
from aniline and nitrobenzene at the cost of approximately
133 kJ/mol and 102 kJ/mol, respectively. These two processes
lead to the intermediate structure Ph-NH+Ph-NO+O+H, which is
235 kJ/mol higher in energy. Next, an N–N single bond can be
formed, releasing approximately 34 kJ/mol. The intermediate Ph-
NH-NO-Ph can then lose the nitrogen-bound H and O atoms in a
slightly endothermic stepwise procedure to form the azo product.
Since hydrogen atoms can recombine on Ag(111) with a very low
reaction barrier of <25 kJ/mol,[34] it can be expected that the two
abstracted H atoms recombine to form H2 at any time during the
reaction, which presumably suppresses the formation of other by-
p-decyloxynitrosobenzene on Ag(111) and found
a clean
dimerization at 420 K (Fig. S16).
With these mechanistic indications in hand, we wanted to
gain further insights into the reaction mechanism. Coupling of
aniline and nitrobenzene on Ag(111) to form trans-azobenzene
was studied as a model reaction using DFT calculations. A four-
layered slab model with a calculated lattice constant of 4.07 Å was
used to sample the Ag(111) surface. The calculations were
performed with the PBE functional,[31,32] the wave functions were
expanded in a plane-wave basis with an energy cut off of 500 eV
and a 3x3x1 k-point grid was used to sample the Brillouin Zone
(see SI for further details). 18 structures possibly involved in this
model reaction were calculated on the Ag(111) surface (also see
Fig. S17-S19 and following discussion). These structures range
from the starting compounds (chosen as reference state with a
relative energy of 0 kJ/mol, in Fig. 6 referred to as Ph-NH2+Ph-
NO2) to trans-azobenzene (co-adsorbed with two oxygen atoms
and one H2 molecule, Ph-N=N-Ph+2 O+H2). The resulting energy
profile can be seen in Fig. 6. Please note that no reaction barriers
or transition states were calculated (also see the discussion
following Fig. S19 for further details).[33] Two pathways connecting
products apart from H2 and adsorbed
O
atoms. This
recombination is exothermic by approximately 25 kJ/mol.
Considering that the reactions are performed under UHV
conditions and that the adsorption energy of H2 molecules on
Ag(111) surfaces is very small (<10 kJ/mol), it can be assumed
that newly formed H2-molecules will be quickly and continuously
removed from the equilibrium. Naturally, this enhances the
formation of H2 and will contribute to the driving force of the overall
endothermic on-surface reaction. The gain in entropy resulting
from the desorption of H2 has been identified as driving force in
5
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10.1002/anie.202011858
Angewandte Chemie International Edition
RESEARCH ARTICLE
other on-surface reactions,[35,36] further supporting the formation
and desorption of H2. Contrary to hydrogen, the abstracted
oxygen atoms are observed to remain – at least partially –
adsorbed on the surface, even at elevated temperatures
(Fig. S14). This agrees well with literature, where the
recombinative desorption of O2 on the Ag(111) surface is
observed at temperatures higher than 597 K.[37]
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Conclusion
In summary, on-surface azo group formation was realized
on silver surfaces. STM results reveal that the azo group
formation by nitro-amino redox cross-coupling (path 1) is the most
efficient reaction on the silver surface in comparison with nitro-
nitro (path 2) and amino-amino homocoupling (path 3). XPS
measurements, as well as theoretically and experimentally
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designed molecular precursors, azo polymers were successfully
prepared. Experiments at different surfaces revealed that the
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based on the XPS study and dimerization reactions of possible
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material along N–N bond formation was suggested as the most
likely reaction path. Our work provides a new bottom-up strategy
to synthesize functional materials through on-surface reactions.
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We thank the Deutsche Forschungsgemeinschaft (SFB 858,
TRR 61, FU 299/19, GA2430/1-1, MO 2345/4-1 and AM 460/2-1)
for financial support. H.-Y.G. acknowledges financial support by
NSFC within Grant 21972104 and “1000-Youth Talents Plan”.
X.M. thanks the Alexander-von-Humboldt foundation for financial
support. H.K. wants to thank Dr. Anup Bhunia and Niklas Radhoff
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Conflict of interest
The authors declare no competing financial interests.
Keywords: Azo formation, cross-coupling, STM, surface
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RESEARCH ARTICLE
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Entry for the Table of Contents
Insert graphic for Table of Contents here.
Am-azo-ing on-surface redox coupling: Herein we disclose the on surface redox coupling of arylamines and nitroarenes on Ag(111).
Starting from well-designed monomeric precursors, the cross-coupling yielding azo bridged poly-phenylenes was investigated by
STM and XPS. Further, first steps toward copolymerization were performed with two comonomers. Mechanistic studies and
supportive DFT calculations are provided.
Institute and/or researcher Twitter usernames: @WWU_Muenster @heklanning
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